Purchase membrane filters from Tisch Scientific but choose wisely.
There are four main considerations in choosing the best membrane filters for your applications. They are:
- Is your filter application automated or manual?
- What is the filter’s chemical compatibility?
- Resistance of membrane to fluid contact
- Extractables
- Absorption
- What Effective Filtration Area is needed for your filtration?
- What pore size rating is optimal for sample clean-up? Does the syringe filter need to be resistant to bases, acids, or organic solvents? Is chemical compatibility is a critical consideration when selecting the sample prep syringe filter?
Aqueous Samples
Hydrophilic membrane filters, which have an affinity for water, are preferable when filtering aqueous samples. Examples are Nylon membrane filters, PES membrane filters, and PVDF membrane filters.
Gases and Aggressive Organic Solvents
Hydrophobic membranes repel water and are inert to aggressive organic solvents, making them ideal for gases and organic solvents. Some examples are PTFE syringe filters and PTFE membrane filters.
Choosing the Right Size
The particulate contained within a fluid affects the life of a filter. As particles are removed from the fluid, they block pores and reduce the useable portion of the filter. Particulate-laden fluids generally plug a filter more quickly than “clean” fluids. Increasing the Effective Filtration Area can lengthen the life of a filter. Filters come in several sizes. 13mm, 17mm, 25mm, and 30mm are available in a variety of membrane and pore size choices.
Another aspect of choosing the right filter size is the hold-up volume. This is the volume of liquid remaining in the filter after use. A filter with a low hold-up volume is recommended for use with expensive fluids or those with limited availability. Of the four common causes for HPLC column failure (plugging, voids, adsorbed sample, and chemical attack), plugging is the most frequently encountered by analytical chemists or analysts. Injection of samples containing particulate will eventually block the column inlet, cause high column backpressure, and shorten the normal lifetime of the column.
Operations of pump components, injectors, and detectors can be expected to be less troublesome when fluids are filtered. For HPLC applications, the 0.45 μm pore size filter is typically selected for removal of particulates. Although there are several seemingly equivalent such products on the market, lack of knowledge about the differences between filters leads to more frequent column replacement and extensive operation downtime. Filtration as a preventative maintenance tool for HPLC analyses is well documented.
It is commonly taken for granted that column life will be extended if samples are filtered prior to injection, but the extension of the column life has not been well quantified. It is the intent of this work to demonstrate that filter efficiency must be considered when choosing an HPLC sample-prep filter and that filtration will lengthen the life of a column.
In this paper, retention efficiency of three effectively equivalent 0.45 μm rated membrane filters was examined using 0.45 μm average diameter latex spheres. This work was conducted with latex spheres to offer the best possible reproducibility in both sample preparation and filter efficiency measurements. In order to correlate the retention of spheres to the actual application, the quantitative effect of filtration on HPLC column life was investigated. This involved examining column life without filtration compared to column life when samples were filtered. It should be recognized that extending the column life is dependent on the particulate within the sample and actual column life extension may vary.